An electromagnetic valve using such a solenoid drive device is disclosed in Japanese Patent Application Publication No. H11-287348 (JP H11-287348 A) (Patent Document 1). This electromagnetic valve includes a cylindrical coil (solenoid), a plunger that is capable of moving in the axial direction by current application to the solenoid, and a core as a magnetic body that attracts the plunger according to excitation of the solenoid. A yoke as a magnetic body having a cylindrical portion is provided radially inward of the solenoid. The core has an accommodating portion having a cylindrical inner peripheral surface, and the accommodating portion is placed radially inward of the solenoid. The yoke and the core are arranged next to each other in the moving direction of the plunger (axial direction) such that a cylindrical inner peripheral surface of a cylindrical portion of the yoke and a cylindrical inner peripheral surface of the core serve as sliding surfaces for the plunger. The yoke and the core are fixed to a cylindrical cover as a magnetic body, and the solenoid, the core, the plunger, the yoke, and the cover form a magnetic circuit. The electromagnetic valve is formed so that the core and the yoke have high magnetic resistance at a predetermined position in order to ensure the force that attracts the plunger by the core in this magnetic circuit. Specifically, clearance having high magnetic resistance is provided between the yoke and the core (Patent Document 1: FIGS. 1 and 2, Paragraphs [0018] to [0022], etc.)
A linear solenoid device disclosed in Japanese Patent Application Publication No. 2009-127692 (JP 2009-127692 A) (Patent Document 2) has a member as a nonmagnetic body between a first core corresponding to the core of Patent Document 1 and a second core corresponding to the yoke of Patent Document 1 (Patent Document 2: FIGS. 1 and 5, Paragraphs [0014] to [0017], etc.). That is, the linear solenoid device of Patent Document 2 has the member as the nonmagnetic body instead of the clearance between the core and the yoke of Patent Document 1. Since the member as the nonmagnetic body is provided, the inner peripheral surface of the first core, the inner peripheral surface of the second core, and the inner peripheral surface of the nonmagnetic body serve as a continuous sliding surface, and slidability is improved. Since relative positional accuracy between the first and second cores and relative positional accuracy between the plunger and the cores are also improved, magnetic loss is reduced.
In an electromagnetic actuator disclosed in Japanese Patent Application Publication No. 2000-21628 (JP 2000-21628 A) (Patent Document 3), the first core, the second core, and the nonmagnetic body in Patent Document 2 are formed as a core-integrated sleeve processed from a single ferromagnetic material. In this core-integrated sleeve, a modified part as a portion corresponding to the clearance of Patent Document 1 and the nonmagnetic body of Patent Document 2 is irradiated with a laser beam while being supplied with an austenite producing element (element that produces a nonmagnetic body or a weak magnetic body) such as, e.g., nickel. As a result, the modified part is alloyed into a nonmagnetic body or a weak magnetic body (Patent Document 3: FIG. 2, Paragraphs [0021] and [0022], etc.).
As described above, various structures are proposed in order to improve magnetic performance. The structure of Patent Document 2 described above can easily ensure positional accuracy of the magnetic body and the nonmagnetic body, and can achieve high magnetic performance. However, manufacturing cost is high because three parts are combined. On the other hand, since the structure of Patent Document 3 is formed by a single part, manufacturing cost is lower than the structure of Patent Document 2. However, it is not easy to accurately manage the range of alloys that are produced by laser beam radiation, the content of the austenite producing element, etc., and magnetic performance is less stable than the structure of Patent Document 2.